Messages in this thread | | | Date | Thu, 26 Mar 2015 17:38:45 +0000 | From | Morten Rasmussen <> | Subject | Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant |
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On Wed, Mar 25, 2015 at 06:08:42PM +0000, Vincent Guittot wrote: > On 25 March 2015 at 18:33, Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Mar 24, 2015 at 11:00:57AM +0100, Vincent Guittot wrote: > >> On 23 March 2015 at 14:19, Peter Zijlstra <peterz@infradead.org> wrote: > >> > On Fri, Feb 27, 2015 at 04:54:07PM +0100, Vincent Guittot wrote: > >> > > >> >> + unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu); > >> > > >> >> + sa->running_avg_sum += delta_w * scale_freq > >> >> + >> SCHED_CAPACITY_SHIFT; > >> > > >> > so the only thing that could be improved is somehow making this > >> > multiplication go away when the arch doesn't implement the function. > >> > > >> > But I'm not sure how to do that without #ifdef. > >> > > >> > Maybe a little something like so then... that should make the compiler > >> > get rid of those multiplications unless the arch needs them. > >> > >> yes, it removes useless multiplication when not used by an arch. > >> It also adds a constraint on the arch side which have to define > >> arch_scale_freq_capacity like below: > >> > >> #define arch_scale_freq_capacity xxx_arch_scale_freq_capacity > >> with xxx_arch_scale_freq_capacity an architecture specific function > > > > Yeah, but it not being weak should make that a compile time warn/fail, > > which should be pretty easy to deal with. > > > >> If it sounds acceptable i can update the patch with your proposal ? > > > > I'll stick it to the end, I just wanted to float to patch to see if > > people had better solutions. > > ok. all other methods that i have tried, was removing the optimization > when default arch_scale_freq_capacity was used
Another potential solution is to stay with weak functions but move the multiplication and shift into the arch_scale_*() functions by passing the value we want to scale into the arch_scale_*() function. That way we can completely avoid multiplication and shift in the default case (no arch_scale*() implementations, which is better than what we have today.
The only downside is that for frequency invariance we need three arch_scale_freq_capacity() calls instead of two.
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